Mobile communication system and mobile communication method

ABSTRACT

In a mobile communication system, user data is communicated directly among a plurality of radio terminals without passing through a radio base station. The communication of user data directly performed among the plurality of radio terminals is performed by using a part of radio resources assigned to the mobile communication system. A D2D terminal included in the plurality of radio terminals notifies the radio base station of an identifier of a neighboring terminal located in the neighborhood of the D2D terminal, or the neighboring terminal notifies the radio base station of the identifier of the D2D terminal.

TECHNICAL FIELD

The present invention relates to a mobile communication system in whichuser data is communicated directly among a plurality of radio terminalsand a mobile communication method used in the mobile communicationsystem.

BACKGROUND ART

In recent years, there has been proposed a technique of directlycommunicating user data (data of User-Plane) among a plurality of radioterminals without passing through a radio base station (D2Dcommunication). The communication of user data directly performed amongthe plurality of radio terminals is performed by using a part of radioresources assigned to a mobile communication system. However, in the D2Dcommunication, the communication of control data (C-Plane) is performedvia the radio base station, similarly to a conventional mobilecommunication system.

PRIOR ART DOCUMENT Non-Patent Document

Non-Patent Document 1: 3GPP technical report “TR 22. 803 V0.3.0” May2012

SUMMARY OF THE INVENTION

Furthermore, in order to realize the D2D communication, a radio terminalneeds to discover another radio terminal. The inventors of the presentinvention found that such a process could also be applied not only tothe D2D communication but also to another process.

In a mobile communication system according to the first aspect, userdata is communicated directly among a plurality of radio terminalswithout passing through a radio base station. The communication of userdata directly performed among the plurality of radio terminals isperformed by using a part of radio resources assigned to the mobilecommunication system. A D2D terminal included in the plurality of radioterminals notifies the radio base station of an identifier of aneighboring terminal located in the neighborhood of the D2D terminal, orthe neighboring terminal notifies the radio base station of theidentifier of the D2D terminal.

A mobile communication method according to the second aspect is used ina mobile communication system, in which communication of user data isdirectly performed among a plurality of radio terminals without passingthrough a radio base station. The communication of user data directlyperformed among the plurality of radio terminals is performed by using apart of radio resources assigned to the mobile communication system. Theplurality of radio terminals included in a transmission-side terminaltransmitting the user date and a reception-side terminal receiving theuser data. The mobile communication method comprises: a step ofnotifying the radio base station, of an identifier of a neighboringterminal located in the neighborhood of the D2D terminal, from a D2Dterminal included in the plurality of radio terminals, or a step ofnotifying the radio base station of an identifier of the D2D terminal,from the neighboring terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a mobile communication system 100according to a first embodiment.

FIG. 2 is a diagram illustrating a radio frame according to the firstembodiment.

FIG. 3 is a diagram illustrating a radio resource according to the firstembodiment.

FIG. 4 is a diagram illustrating a case where the first embodiment isapplied.

FIG. 5 is a diagram illustrating a first assignment mode according tothe first embodiment.

FIG. 6 is a diagram illustrating a second assignment mode according tothe first embodiment.

FIG. 7 is a diagram illustrating UE 10A (transmission-side terminal)according to the first embodiment.

FIG. 8 is a diagram illustrating UE 10B (reception-side terminal)according to the first embodiment.

FIG. 9 is a diagram illustrating a radio base station 310 according tothe first embodiment.

FIG. 10 is a diagram illustrating an example of scheduling according tothe first embodiment.

FIG. 11 is a diagram illustrating an example of scheduling according tothe first embodiment.

FIG. 12 is a sequence diagram illustrating the operation of the mobilecommunication system 100 according to the first embodiment.

FIG. 13 is a sequence diagram illustrating the operation of the mobilecommunication system 100 according to a first modification.

FIG. 14 is a sequence diagram illustrating the operation of the mobilecommunication system 100 according to a second modification.

FIG. 15 is a sequence diagram illustrating the operation of the mobilecommunication system 100 according to a third modification.

FIG. 16 is a sequence diagram illustrating the operation of the mobilecommunication system 100 according to a fourth modification.

FIG. 17 is a sequence diagram illustrating the operation of the mobilecommunication system 100 according to a fifth modification.

FIG. 18 is a diagram for explaining a method of specifying distributionof UEs 10 according to a sixth modification.

FIG. 19 is a diagram for explaining a method of specifying distributionof UEs 10 according to a seventh modification.

FIG. 20 is a diagram for explaining a method of specifying distributionof UEs 10 according to an eighth modification.

DESCRIPTION OF THE EMBODIMENT

Hereinafter, a mobile communication system according to an embodiment ofthe present invention will be described with reference to theaccompanying drawings. Note that in the descriptions of the drawingbelow, identical or similar symbols are assigned to identical or similarportions.

It will be appreciated that the drawings are schematically shown and theratio and the like of each dimension are different from the real ones.Accordingly, specific dimensions should be determined in considerationof the explanation below. Of course, among the drawings, the dimensionalrelationship and the ratio may be different.

[Overview of Embodiment]

In a mobile communication system according to the embodiment, user datais communicated directly among a plurality of radio terminals withoutpassing through a radio base station. The communication of user datadirectly performed among the plurality of radio terminals is performedby using a part of radio resources assigned to the mobile communicationsystem. A D2D terminal included in the plurality of radio terminalnotifies the radio base station of an identifier of a neighboringterminal located in the neighborhood of the D2D terminal, or theneighboring terminal notifies the radio base station of an identifier ofthe D2D terminal.

In the embodiment, the D2D terminal notifies the radio base station ofan identifier of a neighboring terminal, or the neighboring terminalnotifies the radio base station of an identifier of the D2D terminal.Thus, the radio base station can specify a relative positionalrelationship between the D2D terminal and the neighboring terminal.Further, the radio base station can specify a distribution of radioterminals present in a cell managed by the radio base station with acertain degree of accuracy. Such a distribution is useful when used inradio resource scheduling, SON (Self Organizing Network), or MDT(Minimization of Drive Tests).

Furthermore, the communication performed directly among the plurality ofradio terminals without passing through the radio base station may becalled D2D communication. The D2D communication is performed by using apart (a D2D radio resource) of the radio resources assigned to themobile communication system. As the D2D radio resource, for example, apart of uplink radio resources is used.

Furthermore, a radio resource used in the communication of the user datain the D2D communication may be assigned by a radio base station or maybe assigned by a radio terminal (a transmission-side terminal or areception-side terminal).

Furthermore, in the first embodiment, the D2D terminal notifies theradio base station of a received power of a signal transmitted from theneighboring terminal, together with the identifier of the neighboringterminal, or the neighboring terminal notifies the radio base station ofthe received power of the signal transmitted from the D2D terminal,together with the identifier of the D2D terminal.

Furthermore, in the first modification, the D2D terminal notifies theradio base station of the identifier of the neighboring terminal inresponse to an instruction received from the radio base station, or theneighboring terminal notifies the radio base station of an identifier ofthe D2D terminal in response to an instruction received from the radiobase station.

Furthermore, in the second modification, the D2D terminal notifies theradio base station of the received power of the signal transmitted fromthe neighboring terminal in response to a result of comparison betweenthe received power of the signal transmitted from the neighboringterminal and a threshold value, or the neighboring terminal notifies theradio base station of the received power of the signal transmitted fromthe D2D terminal in response to a result of comparison between thereceived power of the signal transmitted from the D2D terminal and athreshold value.

Furthermore, in the second modification, the D2D terminal is notified,from the radio base station, of the threshold value compared with thereceived power of the signal transmitted from the neighboring terminal,or the neighboring terminal is notified, from the radio base station, ofthe threshold value compared with the received power of the signaltransmitted from the D2D terminal.

Furthermore, in the third modification, the D2D terminal is notified,from the radio base station, of a cycle in which the identifier of theneighboring terminal is notified to the radio base station, or theneighboring terminal is notified, from the radio base station, of acycle in which the identifier of the D2D terminal is notified to theradio base station.

Furthermore, in the first embodiment, the radio base station or a nodehigher than the radio base station specifies a distribution of radioterminals present in a cell managed by the radio base station on thebasis of the identifier of the neighboring terminal notified from theD2D terminal or the identifier of the D2D terminal notified from theneighboring terminal.

Furthermore, in the seventh modification, the radio base station or anode higher than the radio base station specifies the distribution ofradio terminals present in the cell managed by the radio base station onthe basis of the identifier of the neighboring terminal notified fromthe D2D terminal and a received power of a signal transmitted from theneighboring terminal or the identifier of the D2D terminal notified fromthe neighboring terminal and a received power of a signal transmittedfrom the D2D terminal.

Furthermore, in the sixth and eighth modification, the radio basestation or a node higher than the radio base station specifies thedistribution of radio terminals present in the cell managed by the radiobase station on the basis of the identifier of the neighboring terminalnotified from the D2D terminal and Timing Advance of the D2D terminal orthe identifier of the D2D terminal notified from the neighboringterminal and Timing Advance of the neighboring terminal.

Furthermore, in a mobile communication system according to theembodiment, user data is communicated directly among a plurality ofradio terminals without passing through a radio base station. Thecommunication of user data directly performed among the plurality ofradio terminals is performed by using a part of radio resources assignedto the mobile communication system. The radio base station is notifiedof an identifier of a neighboring terminal located in the neighborhoodof a D2D terminal from the D2D terminal included in the plurality ofradio terminals or an identifier of the D2D terminal from theneighboring terminal. The radio base station controls a D2D terminalradio resource to be assigned to the D2D terminal or a neighboringterminal radio resource to be assigned to the neighboring terminal onthe basis of the identifier of the neighboring terminal or theidentifier of the D2D terminal.

In the embodiment, the radio base station controls a D2D terminal radioresource to be assigned to the D2D terminal or a neighboring terminalradio resource to be assigned to the neighboring terminal on the basisof the identifier of the neighboring terminal or the identifier of theD2D terminal. Thus, it is possible to suppress interference to thecommunication performed by the neighboring terminal due to thecommunication that the D2D terminal performs directly without passingthrough the radio base station or the interference to the communicationthat the D2D terminal performs directly without passing through theradio base station due to the communication performed by the neighboringterminal.

Furthermore, in the first embodiment, the radio base station is notifiedof a D2D terminal received power of a signal transmitted from theneighboring terminal, together with the identifier of the neighboringterminal from the D2D terminal, or a neighboring terminal received powerof a signal transmitted from the D2D terminal, together with theidentifier of the D2D terminal from the neighboring terminal, and theradio base station controls the D2D terminal radio resource or theneighboring terminal radio resource on the basis of the identifier ofthe neighboring terminal and the D2D terminal received power or theidentifier of the D2D terminal and the neighboring terminal receivedpower.

Furthermore, in the first embodiment, the radio base station controlsthe D2D terminal radio resource or the neighboring terminal radioresource so that the D2D terminal radio resource and the neighboringterminal radio resource do not overlap when it is determined that adistance between the D2D terminal and the neighboring terminal issmaller than a threshold value.

Furthermore, in the first embodiment, the radio base station controlsthe D2D terminal radio resource or the neighboring terminal radioresource so that the D2D terminal radio resource and the neighboringterminal radio resource overlap when it is determined that a distancebetween the D2D terminal and the neighboring terminal is larger than athreshold value.

Furthermore, in the first embodiment, the neighboring terminalcommunicates the user data via the radio base station, and the radiobase station controls the D2D terminal radio resource or the neighboringterminal radio resource so that the D2D terminal radio resource and theneighboring terminal radio resource do not overlap when it is determinedthat a distance between the radio base station and the neighboringterminal is larger than a distance between the radio base station andthe D2D terminal.

Furthermore, in the first embodiment, the neighboring terminalcommunicates the user data via the radio base station, and the radiobase station controls the D2D terminal radio resource or the neighboringterminal radio resource so that the D2D terminal radio resource and theneighboring terminal radio resource overlap when it is determined that adistance between the radio base station and the neighboring terminal issmaller than a distance between the radio base station and the D2Dterminal.

Furthermore, in the fifth modification, the radio base station notifiesanother radio base station different from the radio base station of theidentifier of the neighboring terminal or the identifier of the D2Dterminal.

Furthermore, in the fifth modification, the radio base station notifiesthe other radio base station of the D2D terminal received power,together with the identifier of the neighboring terminal, or theneighboring terminal received power, together with the identifier of theD2D terminal.

Furthermore, a mobile communication method according to the embodimentis a mobile communication method used in a mobile communication system,in which communication of user data is directly performed among aplurality of radio terminals without passing through a radio basestation. The communication of user data directly performed among theplurality of radio terminals is performed by using a part of radioresources assigned to the mobile communication system. The mobilecommunication method comprises: a step of notifying the radio basestation, of an identifier of a neighboring terminal located in theneighborhood of the D2D terminal, from a D2D terminal included in theplurality of radio terminals, or a step of notifying the radio basestation of an identifier of the D2D terminal, from the neighboringterminal.

The mobile communication method according to the embodiment comprises: astep of controlling, at the radio base station, a D2D terminal radioresource to be assigned to the D2D terminal or a neighboring terminalradio resource to be assigned to the neighboring terminal on the basisof the identifier of the neighboring terminal or the identifier of theD2D terminal.

[First Embodiment]

(Mobile Communication System)

Hereinafter, a mobile communication system according to a firstembodiment will be described. FIG. 1 is a diagram illustrating a mobilecommunication system 100 according to the first embodiment.

As illustrated in FIG. 1, the mobile communication system 100 includes aradio terminal 10 (hereinafter, referred to as UE 10) and a core network50. Furthermore, the mobile communication system. 100 includes a firstcommunication system and a second communication system.

The first communication system is a communication system correspondingto LTE (Long Term Evolution), for example. The first communicationsystem has a base station 110A (hereinafter, referred to as MeNB 110A),a home base station 110B (hereinafter, referred to as HeNB 110B), a homebase station gateway 120B (hereinafter, referred to as HeNB-GW 120B),and MME 130, for example.

In addition, a radio access network (E-UTRAN; Evolved UniversalTerrestrial Radio Access Network) corresponding to the firstcommunication system is configured by the MeNB 110A, the HeNB 110B, andthe HeNB-GW 120B.

The second communication system is a communication system correspondingto UMTS (Universal Mobile Telecommunication System), for example. Thesecond communication system includes a base station 210A (hereinafter,referred to as MNB 210A), a home base station 210B (hereinafter,referred to as HNB 210B), RNC 220A, a home base station gateway 220B(hereinafter, referred to as HNB-GW 220B), and SGSN 230.

In addition, a radio access network (UTRAN; Universal Terrestrial RadioAccess Network) corresponding to the second communication system isconfigured by the MNB 210A, the HNB 210B, the RNC 220A, and the HNB-GW220B.

The UE 10 is a device (User Equipment) that communicates with the secondcommunication system or the first communication system. For example, theUE 10 has a function of performing radio communication with the MeNB110A and the HeNB 110B. Alternatively, the UE 10 has a function ofperforming radio communication with the MNB 210A and the HNB 210B.

The MeNB 110A, which manages a general cell 111A, is a device (evolvedNodeB) that performs radio communication with the UE 10 being present inthe general cell 111A.

The HeNB 110B, which manages a specific cell 111B, is a device (Homeevolved NodeB) that performs radio communication with the UE 10 beingpresent in the specific cell 111B.

The HeNB-GW 120B, which is connected to the HeNB 110B, is a device (Homeevolved NodeB Gateway) that manages the HeNB 110B.

The MME 130, which is connected to the MeNB 110A, is a device (MobilityManagement Entity) that manages the mobility of the UE 10 having set upa radio connection with the MeNB 110A. Furthermore, the MME 130, whichis connected to the HeNB 110B via the HeNB-GW 120B, is a device thatmanages the mobility of the UE 10 having set up a radio connection withthe HeNB 110B.

The MNB 210A, which manages a general cell 211A, is a device (NodeB)that performs radio communication with the UE 10 being present in thegeneral cell 211A.

The HNB 210B, which manages a specific cell 211B, is a device (HomeNodeB) that performs radio communication with the UE 10 being present inthe specific cell 211B.

The RNC 220A, which is connected to the MNB 210A, is a device (RadioNetwork Controller) that sets up a radio connection (RRC Connection)with the UE 10 being present in the general cell 211A.

The HNB-GW 220B, which is connected to the HNB 210B, is a device (HomeNodeB Gateway) that sets up a radio connection (RRC Connection) with theUE 10 being present in the specific cell 211B.

The SGSN 230 is a device (Serving GPRS Support Node) that performspacket switching in a packet switching domain. The SGSN 230 is providedin the core network 50. Although not illustrated in FIG. 1, a device(MSC; Mobile Switching Center) that performs circuit switching in acircuit switching domain may be provided in the core network 50.

In addition, it is noted that the general cell and the specific cell areunderstood as a function of performing radio communication with the UE10. However, the general cell and the specific cell are also used as aterm indicating a coverage area of a cell. Furthermore, cells such asgeneral cells and specific cells are identified by frequencies,spreading codes, time slots and the like used in the cells.

Here, a coverage area of the general cell is wider than a coverage areaof the specific cell. The general cell, for example, is a macro cellprovided by a communication provider. The specific cell, for example, isa femto cell or a home cell provided by the third party other than thecommunication provider. The specific cell may be a CSG (ClosedSubscriber Group) cell or a pico cell provided by the communicationprovider.

Hereinafter, the first communication system will be mainly described.The following description may also be applied to the secondcommunication system.

In the first communication system, an OFDMA (Orthogonal FrequencyDivision Multiple Access) scheme is used as a downlink multiplexingscheme, and an SC-FDMA (Single-Carrier Frequency Division MultipleAccess) scheme is used as an uplink multiplexing scheme.

Furthermore, in the first communication system, as an uplink channel, anuplink control channel (PUCCH; Physical Uplink Control Channel), anuplink shared channel (PUSCH; Physical Uplink Shared Channel) and thelike exist. Furthermore, as a downlink channel, a downlink controlchannel (PDCCH; Physical Downlink Control Channel), a downlink sharedchannel (PDSCH; Physical Downlink Shared Channel) and the like exist.

The uplink control channel is a channel that carries a control signal.The control signal, for example, includes CQI (Channel QualityIndicator), PMI (Precoding Matrix Indicator), RI (Rank Indicator), SR(Scheduling Request), and ACK/NACK.

The CQI is a signal for notifying of a recommended modulation scheme anda coding rate to be used in the downlink transmission. The PMI is asignal that indicates a precoding matrix that is desirably used for thedownlink transmission. The RI is a signal that indicates the number oflayers (the number of streams) to be used in the downlink transmission.The SR is a signal that requests the assignment of an uplink radioresource (a resource block which will be described later). The ACK/NACKis a signal that indicates whether a signal transmitted through adownlink channel (for example, PDSCH) has been received.

The uplink shared channel is a channel that carries a control signal(including the aforementioned control signal) and/or a data signal. Forexample, the uplink radio resource may be assigned only to the datasignal, or may be assigned such that the data signal and the controlsignal are multiplexed.

The downlink control channel is a channel that carries a control signal.The control signal, for example, is Uplink Scheduling Grants, DownlinkScheduling Assignments, and TPC bit.

The Uplink SI is a signal that indicates the assignment of the uplinkradio resource. The Downlink SI is a signal that indicates theassignment of a downlink radio resource. The TPC bit is a signal thatindicates increase and decrease in power of a signal that is transmittedthrough the uplink channel.

The downlink shared channel is a channel that carries a control signaland/or a data signal. For example, the downlink radio resource may beassigned only to the data signal, or may be assigned such that the datasignal and the control signal are multiplexed.

In addition, a control signal transmitted through the downlink sharedchannel includes TA (Timing Advance). The TA is transmission timingcorrection information between the UE 10 and the MeNB 110A, and ismeasured by the MeNB 110A on the basis of an uplink signal transmittedfrom the UE 10.

Furthermore, a control signal that is transmitted through a channelother than the downlink control channel (PDCCH) and the downlink sharedchannel (PDSCH) includes the ACK/NACK. The ACK/NACK is a signal thatindicates whether a signal transmitted through an uplink channel (forexample, PUSCH) has been received.

In addition, the general cell and the specific cell broadcastinformation through a broadcast channel (BCCH; Broadcast ControlChannel). The broadcast information, for example, is information such asMIB (Master Information Block) and SIB (System Information Block).

Specifically not illustrated in FIG. 1, the first communication systemmay include a relay node that relays data communication between the MeNB110A (or the HeNB 110B) and the UE 10. Similarly, the secondcommunication system may include a relay node that relays datacommunication with the MNB 210A (or the HNB 210B).

(Radio Frame)

Hereinafter, a radio frame in the first communication system will bedescribed. FIG. 2 is a diagram illustrating the radio frame in the firstcommunication system.

As illustrated in FIG. 2, one radio frame is configured by 10 subframesand one subframe is configured by two slots. One slot has a time lengthof 0.5 msec, one subframe has a time length of 1 msec, and one radioframe has a time length of 10 msec.

In addition, one slot is configured by a plurality of OFDM symbols (forexample, six OFDM symbols or seven OFDM symbols) in a downlink.Similarly, one slot is configured by a plurality of SC-FDMA symbols (forexample, six SC-FDMA symbols or seven SC-FDMA symbols) in an uplink.

(Radio Resource)

Hereinafter, a radio resource in the first communication system will bedescribed. FIG. 3 is a diagram illustrating the radio resource in thefirst communication system.

As illustrated in FIG. 3, a radio resource is defined by a frequencyaxis and a time axis. A frequency is configured by a plurality ofsubcarriers, and a predetermined number of subcarriers (12 subcarriers)are collectively called a resource block (RB). A time has a unit, suchas the OFDM symbol (or the SC-FDMA symbol), the slot, the subframe, andthe radio frame, as described above.

Here, the radio resource is assignable to each one resource block.Furthermore, on the frequency axis and the time axis, it is possible todivide and assign the radio resources to a plurality of users (forexample, a user #1 to a user #5).

Furthermore, the radio resource is assigned by the MeNB 110A. The MeNB110A assigns the radio resources to each UE 10 on the basis of the CQI,the PMI, the RI and the like.

(Application Case)

Hereinafter, the application case according to the first embodiment willbe described. FIG. 4 is a diagram for explaining the application caseaccording to the first embodiment. In FIG. 4, UE 10A and UE 10B areillustrated as the UE 10. A radio base station 310 is preferably theMeNB 110A or the HeNB 110B. However, the radio base station 310 may bethe MNB 210A or the HNB 210B. Alternatively, the radio base station 310may be a relay node. A network device 330 is a device provided in thecore network 50. The network device 330 may be the MME 130 or the SGSN230.

As illustrated in FIG. 4, the communication of the user data (data ofUser-Plane) is directly performed among a plurality of UEs 10 withoutpassing through the radio base station (hereinafter, referred to as D2Dcommunication). On the other hand, the communication of the control data(data of C-Plane) is performed via the radio base station 310, similarlyto a conventional mobile communication system.

Furthermore, the D2D communication is performed by using a part(hereinafter, a D2D radio resource) of radio resources assigned to themobile communication system. As the D2D radio resource, for example, apart of uplink radio resources is used. A radio resource used in the D2Dcommunication of the user data may be assigned by the radio base station310 or may be assigned by the UE 10 (a transmission-side terminal or areception-side terminal).

The D2D radio resource is preferably broadcast from respective cellsmanaged by the radio base station 310, for example. The D2D radioresource may be included in MIB (Master Information Block) or SIB(System Information Block), for example.

Furthermore, each UE 10 broadcasts a signal (hereinafter, referred to asDiscovery signal) with predetermined power for detecting a neighboringterminal capable of performing D2D communication. On the other hand, theneighboring terminal broadcasts a response signal (hereinafter, referredto as Discovery response) with predetermined power to the Discoverysignal. The Discovery signal includes an identifier (an identifier ofthe UE 10) of a transmission source of the Discovery signal. TheDiscovery response includes an identifier (an identifier of theneighboring terminal) of the transmission source of the Discoveryresponse.

(First Assignment Mode)

Hereinafter, a first assignment mode according to the first embodimentwill be described. FIG. 5 is a diagram for explaining the firstassignment mode according to the first embodiment. In FIG. 5, as the UE10, the UE 10A and UE 10B are illustrated. The UE 10A is an example of atransmission-side terminal and the UE 10B is an example of areception-side terminal. In the embodiment, a case where a secondassignment mode is used will be mainly described, a scene where thefirst assignment mode is used may be present.

As illustrated in FIG. 5, in the first assignment mode, the UE 10 (theUE 10A or UE 10B) assigns the radio resource used for the communicationof the user data in the D2D communication. Specifically, the UE 10 (theUE 10A or UE 10B) autonomously assigns the radio resource used for thecommunication of the user data in the D2D communication from among D2Dradio resources broadcast from each cell managed by the radio basestation 310. The UE 10 notifies another UE 10 of the assigned radioresource (resource assignment).

The UE 10 may directly notify another UE 10 of the assigned radioresource or may notify the other UE 10 via the radio base station 310and without passing through the core network.

The UE 10A transmits user data to the UE 10B by using the assigned radioresource. Similarly, the UE 10B receives the user data from the UE 10Aby using the assigned radio resource.

(Second Assignment Mode)

Hereinafter, the second assignment mode according to the firstembodiment will be described. FIG. 6 is a diagram for explaining thesecond assignment mode according to the first embodiment. In FIG. 6, asthe UE 10, the UE 10A and UE 10B are illustrated. The UE 10A is anexample of a transmission-side terminal and the UE 10B is an example ofa reception-side terminal.

As illustrated in FIG. 6, in the second assignment mode, the radio basestation 310 assigns the radio resource used for the communication of theuser data in the D2D communication. Specifically, the radio base station310 assigns, to the UE 10A and UE 10B, the radio resource used for thecommunication of the user data in the D2D communication from among D2Dradio resources. The radio base station 310 notifies the UE 10A and UE10B of the assigned radio resource (resource assignment).

The UE 10A transmits user data to the UE 10B by using the assigned radioresource. Similarly, the UE 10B receives the user data from the UE 10Aby using the assigned radio resource.

Specifically, the radio base station 310 notifies the UE 10A and UE 10Bof a target received power of the user data used in the D2Dcommunication before starting the D2D communication. The radio basestation 310 may broadcast the target received power by using a broadcastchannel such as SIB and may transmit the target received power to the UE10A and UE 10B by using an individual control channel such as PDCCH. Thetarget received power is also called “Nominal Power for D2D”.

In the embodiment, the radio base station 310 notifies the UE 10A (thetransmission-side terminal) of the assignment information on the radioresource used in the D2D communication. When the D2D communication isperformed by using an uplink radio resource, the assignment informationof the radio resource is the existing uplink scheduling information usedin the cellular communication. The transmission unit 314 transmits theuplink scheduling information to the UE 10A by using the individualcontrol channel such as PDCCH. The uplink scheduling information is alsocalled “Uplink Scheduling Grants”. However, it is to be noted that theuplink scheduling information is extended as below.

Here, the uplink scheduling information includes at least one of:information indicating the uplink radio resource assigned to the UE 10A;information indicating whether a transmission acknowledgment signalindicating whether the user data transmitted from the UE 10A has beenable to be received is notified via the radio base station 310;information indicating a power control method used in D2D communication;and information indicating received power generated when the UE 10Ereceives the user data transmitted from the UE 10A.

In the embodiment, the radio base station 310 notifies the UE 10B (thereception-side terminal) of the assignment information of the radioresource used in the D2D communication. When the D2D communication isperformed by using the uplink radio resource, the assignment informationof the radio resource is D2D scheduling information that is differentfrom the existing uplink scheduling information used in the cellularcommunication. It is to be noted that the D2D scheduling information isscheduling information used in the D2D communication. The radio basestation 310 transmits the D2D scheduling information to the UE 10B byusing the individual control channel such as PDCCH. The D2D schedulinginformation is also called “D2D Scheduling Grants”.

Here, the D2D scheduling information includes at least one of:information indicating that the uplink radio resource is used as areception resource of D2D communication; the identifier of the UE 10A;information indicating the uplink radio resource assigned to the UE 10A;information indicating whether a transmission acknowledgment signalindicating whether the user data transmitted from the UE 10A has beenable to be received is notified via the radio base station 310; andinformation indicating a power control method used in the D2Dcommunication.

However, the radio base station 310 may assign radio resources, at onetime, to the UE 10A and UE 10B by using a single control signal by usingRNTI that is common to the UE 10A and UE 10B rather than separatelyusing the uplink scheduling information and D2D scheduling information.

(Transmission-Side Terminal)

Hereinafter, the transmission-side terminal according to the firstembodiment will be described. Furthermore, as the transmission-sideterminal, the UE 10A is illustrated. FIG. 7 is a block diagramillustrating the UE 10A according to the first embodiment.

As illustrated in FIG. 7, the UE 10A includes a reception unit 13A, atransmission unit 14A, and a control unit 15A.

The reception unit 13A receives data from the radio base station 310 inthe communication performed with the radio base station 310(hereinafter, referred to as cellular communication). The reception unit13A receives data from the UE 10B in the D2D communication. For example,in the D2D communication, the reception unit 13A may receive, from theUE 10B, a transmission acknowledgment signal (ACK/NACK) indicatingwhether user data has been able to be received. The reception unit 13Amay receive the transmission acknowledgment signal via the radio basestation 310 in the D2D communication.

In the embodiment, the reception unit 13A receives a Discovery responsefrom a neighboring terminal located in the neighborhood of the UE 10A.As described above, the Discovery response includes an identifier of atransmission source (the neighboring terminal) of the Discoveryresponse.

The transmission unit 14A transmits data to the radio base station 310in the cellular communication. The transmission unit 14A transmits datato the UE 10B in the D2D communication. For example, the transmissionunit 14A transmits user data to the UE 10B in the D2D communication.Furthermore, the transmission unit 14A retransmits the user data to theUE 10B in response to an instruction output from the control unit 15A.

In the first embodiment, the transmission unit 14A may transmit a D2Dcontrol signal for controlling direct communication of user data to theradio base station 310.

The D2D control signal, for example, indicates at least one of: a signalfor requesting the switching of the first assignment mode and the secondassignment mode; a signal indicating that transmission power used forthe communication of the user data has exceeded a threshold value; asignal indicating that the transmission power used for the communicationof the user data has become less than the threshold value; a signalindicating that a modulation and coding scheme used for thecommunication of the user data has become less than the threshold value;and a signal indicating that the modulation and coding scheme used forthe communication of the user data has exceeded the threshold value.

In the embodiment, the transmission unit 14A transmits the Discoverysignal with predetermined power. As described above, the Discoverysignal includes an identifier of a transmission source (the UE 10A) ofthe Discovery signal. Moreover, the transmission unit 14A transmits theidentifier of the neighboring terminal to the radio base station 310 inresponse to reception of the Discovery response. The transmission unit14A may transmit information indicating received power of the signal(for example, the Discovery response) transmitted from the neighboringterminal to the radio base station 310, together with the identifier ofthe neighboring terminal. The received power of the signal transmittedfrom the neighboring terminal is naturally measured by the UE 10A.

The control unit 15A controls the UE 10A. Specifically, the control unit15A determines a change in the communication state of the user datacommunicated directly between the UE 10A and UE 10B.

Here, the case in which it is determined that the communication state isimproved indicates the case in which the transmission power used for thecommunication of the user data has become less than the threshold valueor the modulation and coding scheme used for the communication of theuser data has exceeded the threshold value. Alternatively, the case inwhich it is determined that the communication state is improved mayindicate the case in which a block error rate has become less than thethreshold value, the case in which a packet error rate has become lessthan the threshold value, the case in which predetermined QoS has beensatisfied, the case in which CQI has exceeded the threshold value, andthe case in which a processing load of the UE 10A has become less thanthe threshold value.

Furthermore, the case in which it is determined that the communicationstate is deteriorated indicates the case in which the transmission powerused for the communication of the user data has exceeded the thresholdvalue or the modulation and coding scheme used for the communication ofthe user data has become less than the threshold value. Alternatively,the case in which it is determined that the communication state isdeteriorated may indicate the case in which the block error rate hasexceeded the threshold value, the case in which the packet error ratehas exceeded the threshold value, the case in which the predeterminedQoS has not been satisfied, the case in which the CQI has become lessthan the threshold value, and the case in which the processing load ofthe UE 10A has exceeded the threshold value.

Furthermore, when it is determined that the communication state isdeteriorated, the control unit 15A instructs the transmission unit 14Ato transmit the D2D control signal. When it is determined that thecommunication state is improved, the control unit 15A instructs thetransmission unit 14A to transmit the D2D control signal.

In the first embodiment, when it is determined that the communicationstate is deteriorated, the assignment mode is switched from the firstassignment mode to the second assignment mode by the transmission of theD2D control signal. Meanwhile, when it is determined that thecommunication state is improved, the assignment mode is switched fromthe second assignment mode to the first assignment mode by thetransmission of the D2D control signal.

(Reception-Side Terminal)

Hereinafter, the reception-side terminal according to the firstembodiment will be described. Furthermore, as the transmission-sideterminal, the UE 10B is illustrated. FIG. 8 is a block diagramillustrating the UE 10B according to the first embodiment.

As illustrated in FIG. 8, the UE 10B includes a reception unit 13B, atransmission unit 14B, and a control unit 15B.

The reception unit 13B receives data from the radio base station 310 inthe cellular communication. The reception unit 13B receives data fromthe UE 10A in the D2D communication. For example, in the D2Dcommunication, the reception unit 13B receives user data (initialtransmission) transmitted from the UE 10A. Furthermore, the transmissionunit 14A receives user data (retransmission) retransmitted from the UE10A.

In the embodiment, the reception unit 13B receives the Discoveryresponse from the neighboring terminal located in the neighborhood ofthe UE 10B. As described above, the Discovery response includes anidentifier of a transmission source (the neighboring terminal) of theDiscovery response.

The transmission unit 14B transmits data to the radio base station 310in the cellular communication. The transmission unit 14B transmits datato the UE 10A in the D2D communication. For example, the transmissionunit 14B may transmit, to the UE 10A, a transmission acknowledgmentsignal (ACK/NACK) indicating whether user data has been able to bereceived. The transmission unit 14B may transmit a transmissionacknowledgment signal (ACK/NACK) to the user data in the D2Dcommunication to the radio base station 310.

In the embodiment, the transmission unit 14B may transmit a D2D controlsignal for controlling direct communication of the user data to theradio base station 310. Furthermore, it is sufficient if the D2D controlsignal is transmitted to the radio base station 310 from at least one ofthe UE 10A and UE 10B.

In the embodiment, the transmission unit 14B transmits the Discoverysignal with predetermined power. As described above, the Discoverysignal includes an identifier of a transmission source (the UE 10B) ofthe Discovery signal. Moreover, the transmission unit 14B transmits anidentifier of the neighboring terminal to the radio base station 310 inresponse to reception of the Discovery response. The transmission unit14B may transmit information indicating the received power of the signal(for example, the Discovery response) transmitted from the neighboringterminal to the radio base station 310, together with the identifier ofthe neighboring terminal. The received power of the signal transmittedfrom the neighboring terminal is naturally measured by the UE 10B.

The control unit 15B controls the UE 10B. Specifically, similarly to thecontrol unit 15A, the control unit 15B determines a change in thecommunication state of the user data communicated directly between theUE 10A and UE 10B. Similarly to the control unit 15A, when it isdetermined that the communication state is deteriorated, the controlunit 15B instructs the transmission unit 14B to transmit the D2D controlsignal. Alternatively, similarly to the control unit 15A, when it isdetermined that the communication state is improved, the control unit15B instructs the transmission unit 14B to transmit the D2D controlsignal.

As described above, when it is determined that the communication stateis deteriorated, the assignment mode is switched from the firstassignment mode to the second assignment mode by the transmission of theD2D control signal. Meanwhile, when it is determined that thecommunication state is improved, the assignment mode is switched fromthe second assignment mode to the first assignment mode by thetransmission of the D2D control signal.

(Radio Base Station)

Hereinafter, the radio base station according to the first embodimentwill be described. FIG. 9 is a block diagram illustrating the radio basestation 310 according to the first embodiment.

As illustrated in FIG. 9, the radio base station 310 includes areception unit 313, a transmission unit 314, and a control unit 315.

The reception unit 313 receives data from the UE 10. For example, in theD2D communication, the reception unit 313 receives, from the UE 10B, atransmission acknowledgment signal (ACK/NACK) indicating whether userdata has been able to be received. Furthermore, the reception unit 313may receive the user data transmitted from the UE 10A to the UE 10B.

In the embodiment, the reception unit 313 receives a D2D control signalfrom the UE 10 (the UE 10A or UE 10B). Moreover, the reception unit 313receives the identifier of the neighboring terminal located in theneighborhood of the UE 10 (the UE 10A or UE 10B). The reception unit 313may receive information indicating the received power of the signal (forexample, the Discovery response) transmitted from the neighboringterminal, together with the identifier of the neighboring terminal. Thereceived power of the signal transmitted from the neighboring terminalis naturally measured by the UE 10 (the UE 10A or UE 10B).

The transmission unit 314 receives data from the UE 10. For example,when the transmission acknowledgment signal (ACK/NACK) is received fromthe UE 10B in the D2D communication, in response to the transmissionacknowledgment signal (ACK/NACK), the transmission unit 314 transmitsthe transmission acknowledgment signal to the UE 10A. For example, thetransmission unit 314 may relay the transmission acknowledgment signal,which is received from the UE 10B, to the UE 10A. Alternatively, thetransmission unit 314 may transmit the transmission acknowledgmentsignal to the UE 10A, together with a signal for assigning a radioresource used for the communication of user data in the D2Dcommunication to the UE 10A.

The control unit 315 controls the radio base station 310. Specifically,the control unit 315 assigns uplink and downlink radio resources to theUE 10. Furthermore, the control unit 315 may assign the radio resource,which is assigned to the communication of the user data communicateddirectly between the UE 10A and UE 10B, as the reception resource forreceiving the user data. In this way, the radio base station 310 is ableto receive the user data communicated directly between the UE 10A and UE10B.

In the first embodiment, in response to the D2D control signal, thecontrol unit 315 switches the first assignment mode and the secondassignment mode. For example, in response to the reception of the D2Dcontrol signal indicating that the communication state of the user datacommunicated directly between the UE 10A and UE 10B is deteriorated, thecontrol unit 315 switches the assignment mode from the first assignmentmode to the second assignment mode. That is, the control unit 315assigns a radio resource for receiving the user data in the D2Dcommunication to the UE 10A and UE 10B.

Meanwhile, in response to the reception of the D2D control signalindicating that the communication state of the user data communicateddirectly between the UE 10A and UE 10B is improved, the control unit 315switches the assignment mode from the second assignment mode to thefirst assignment mode. That is, the control unit 315 does notparticipate in assigning the radio resource for receiving the user datain the D2D communication.

In the embodiment, in the second assignment mode, the control unit 315assigns a D2D radio resource to be assigned to the UE 10 or aneighboring terminal radio resource to be assigned to the neighboringterminal on the basis of the identifier of the neighboring terminalreceived from the UE 10 (the UE 10A or UE 10B). In the second assignmentmode, the control unit 315 may assign the D2D radio resource to beassigned to the UE 10 or the neighboring terminal radio resource to beassigned to the neighboring terminal on the basis of the received powerof the signal (for example, the Discovery response) transmitted from theneighboring terminal and the identifier of the neighboring terminal.

Specifically, when it is determined that the distance between the UE 10(the D2D terminal) and the neighboring terminal is smaller than athreshold value, the control unit 315 controls the D2D terminal radioresource or the neighboring terminal radio resource so that the D2Dterminal radio resource and the neighboring terminal radio resource donot overlap.

Here, the control unit 315 may determine that the distance between theUE 10 and the neighboring terminal is smaller than the threshold valuewhen the identifier of the neighboring terminal is received.Alternatively, the control unit 315 may determine that the distancebetween the UE 10 and the neighboring terminal is smaller than thethreshold value when the identifier of the neighboring terminal isreceived and the received power of the signal (for example, theDiscovery response) transmitted from the neighboring terminal is largerthan a threshold value.

Alternatively, the control unit 315 controls the D2D terminal radioresource or the neighboring terminal radio resource so that the D2Dterminal radio resource and the neighboring terminal radio resourceoverlap when the neighboring terminal performs D2D communication and itis determined that the distance between the UE 10 (the D2D terminal) andthe neighboring terminal is larger than the threshold value.

Furthermore, the control unit 315 may determine that the distancebetween the UE 10 and the neighboring terminal is larger than thethreshold value when the identifier of the neighboring terminal is notreceived. Alternatively, the control unit 315 may determine that thedistance between the UE 10 and the neighboring terminal is larger thanthe threshold value when the identifier of the neighboring terminal isreceived and the received power of the signal (for example, theDiscovery response) transmitted from the neighboring terminal is smallerthan the threshold value.

Alternatively, the control unit 315 may control the D2D terminal radioresource or the neighboring terminal radio resource so that the D2Dterminal radio resource and the neighboring terminal radio resource donot overlap when the neighboring terminal performs cellularcommunication and it is determined that the distance between theneighboring terminal and the radio base station 310 is larger than thedistance between the UE 10 (the D2D terminal) and the radio base station310.

Alternatively, the control unit 315 may control the D2D terminal radioresource or the neighboring terminal radio resource so that the D2Dterminal radio resource and the neighboring terminal radio resourceoverlap when the neighboring terminal performs cellular communicationand it is determined that the distance between the neighboring terminaland the radio base station 310 is smaller than the distance between theUE 10 (the D2D terminal) and the radio base station 310.

(Example of Scheduling)

Hereinafter, an example of scheduling according to the first embodimentwill be described. FIGS. 10 and 11 are diagrams for explaining anexample of scheduling according to the first embodiment.

First, as illustrated in FIG. 10, a case where UE 10 ₁ and UE 10 ₂perform D2D communication, UE 10 ₃ and UE 10 ₄ perform another D2Dcommunication, and UE 10 ₅ performs cellular communication will bedescribed.

In such a case, since the distance between the UE 10 ₁ (the D2Dterminal) and the UE 10 ₅ (the neighboring terminal) is smaller than athreshold value, the radio base station 310 controls radio resources sothat a radio resource (RB2) assigned to the UE 10 ₁ (the D2D terminal)and a radio resource (RB1) assigned to the UE 10 ₅ (the neighboringterminal) do not overlap.

Moreover, since the distance between the UE 10 ₂ (the D2D terminal) andthe UE 10 ₃ (the neighboring terminal) is smaller than the thresholdvalue, the radio base station 310 controls radio resources so that theradio resource (RB2) assigned to the UE 10 ₂ (the D2D terminal) and theradio resource (RB1) assigned to the UE 10 ₃ (the neighboring terminal)do not overlap.

However, since the distance between the UE 10 ₃ (the D2D terminal) orthe UE 10 ₄ (the D2D terminal) and the UE 10 ₅ (the neighboringterminal) is larger than the threshold value, the radio base station 310may control the radio resources so that the radio resource (RB1)assigned to the UE 10 ₃ and the UE 10 ₄ (the D2D terminals) and theradio resource (RB1) assigned to the UE 10 ₅ (the neighboring terminal)overlap.

Second, as illustrated in FIG. 11, a case where the UE 10 ₁ and the UE10 ₂ perform D2D communication, the UE 10 ₃ performs cellularcommunication, the UE 10 ₄ and the UE 10 ₅ perform another D2Dcommunication, and UE 10 ₆ performs cellular communication will bedescribed.

In such a case, since the distance between the UE 10 ₃ (the neighboringterminal) and the radio base station 310 is smaller than the distancebetween the UE 10 ₁ and the UE 10 ₂ (the D2D terminals), and the radiobase station 310, the radio base station 310 may control radio resourcesso that the radio resource assigned to the UE 10 ₁ and the UE 10 ₂ (theD2D terminals) and the radio resource (RB1) assigned to the UE 10 ₃ (theneighboring terminal) overlap.

In such a case, it is considered that since the transmission power usedfor the communication of the UE 10 ₃ (the neighboring terminal) may besmall, even when the above-described radio resource control isperformed, the interference between the communication (D2Dcommunication) of the UE 10 ₁ and the UE 10 ₂ and the communication(cellular communication) of the UE 10 ₃ (the neighboring terminal) maybe suppressed.

On the other hand, since the distance between the UE 10 ₆ (theneighboring terminal) and the radio base station 310 is larger than thedistance between the UE 10 ₄ and the UE 10 ₅ (the D2D terminals) and theradio base station 310, the radio base station 310 controls the radioresources so that the radio resource assigned to the UE 10 ₄ and the UE10 ₅ (the D2D terminals) and the radio resource (RB1) assigned to the UE10 ₆ (the neighboring terminal) do not overlap.

In such a case, although the transmission power used for thecommunication of the UE 10 ₆ (the neighboring terminal) may be large,the interference between the communication (D2D communication) of the UE10 ₄ and the UE 10 ₅ and the communication (cellular communication) ofthe UE 10 ₆ (the neighboring terminal) is suppressed by theabove-described radio resource control.

(Mobile Communication Method)

Hereinafter, a mobile communication method according to the firstembodiment will be described. FIG. 12 is a diagram illustrating a mobilecommunication method according to the first embodiment. In FIG. 12, itis to be noted that the second assignment mode is used.

As illustrated in FIG. 12, in step 10, the UE 10 detects a neighboringterminal located in the neighborhood of the UE 10. For example, the UE10 broadcasts a signal (the Discovery signal) with predetermined powerfor detecting a neighboring terminal capable of performing D2Dcommunication. The Discovery signal includes an identifier of atransmission source.

When the neighboring terminal is present, the UE 10 receives a Discoveryresponse from the neighboring terminal as a response to the Discoverysignal. The Discovery response includes an identifier of thetransmission source (namely, the neighboring terminal).

In step 20, the UE 10 transmits an identifier of the neighboringterminal to the radio base station 310. The UE 10 may transmitinformation indicating the received power of the signal (for example,the Discovery response) transmitted from the neighboring terminal to theradio base station 310, together with the identifier of the neighboringterminal.

In step 30, the radio base station 310 assigns a D2D radio resource tobe assigned to the UE 10 or a neighboring terminal radio resource to beassigned to the neighboring terminal (scheduling). Specifically, theradio base station 310 performs radio resource assignment illustrated inFIGS. 10 and 11.

(Operation and Effect)

In the first embodiment, the UE 10 (the D2D terminal) can notify theradio base station 310 of an identifier of a neighboring terminal, andthe radio base station 310 can specify a distribution of the UEs 10present in a cell managed by the radio base station 310 with a certaindegree of accuracy. Such a distribution is useful when used in radioresource scheduling, SON (Self Organizing Network), or MDT (Minimizationof Drive Tests).

Moreover, the radio base station 310 controls the D2D terminal radioresource to be assigned to the UE 10 (the D2D terminal) or theneighboring terminal radio resource assigned to be the neighboringterminal on the basis of the identifier of the neighboring terminal.Thus, it is possible to suppress the interference to the communicationperformed by the neighboring terminal due to D2D communication or theinterference to the D2D communication due to the communication performedby the neighboring terminal.

[First Modification]

Hereinafter, a first modification of the first embodiment is explained.In the first modification, the radio base station 310 designates whetherit is necessary to notify of the received power of the signal (forexample, the Discovery response) transmitted from the neighboringterminal. In other words, the radio base station 310 designates whetherthe UE 10 notifies of the received power of the signal transmitted fromthe neighboring terminal.

In the first modification, the UE 10 is configured to notify the radiobase station 310 of the received power of the signal (for example, theDiscovery response) transmitted from the neighboring terminal as adefault operation.

Specifically, as illustrated in FIG. 13, in step 110, the UE 10 detectsa neighboring terminal located in the neighborhood of the UE 10.Specifically, the UE 10 broadcasts a signal (the Discovery signal) withpredetermined power for detecting a neighboring terminal capable ofperforming D2D communication. When the neighboring terminal is present,the UE 10 receives a Discovery response from the neighboring terminal asa response to the Discovery signal.

In step 120, the radio base station 310 notifies the UE 10 of the factthat it is not necessary to notify of the received power (notificationOFF request). For example, when it is desired to suppress overhead(consumption of radio resources) accompanied by notification of thereceived power, the radio base station 310 notifies the UE 10 of thenotification OFF request. The case where overhead is to be suppressed,for example, is a case where a large number of received powernotifications may occur. The notification OFF request is transmittedusing a broadcast channel such as SIB, for example.

In step 130, the UE 10 transmits the identifier of the neighboringterminal to the radio base station 310. However, the UE 10 does nottransmit the information indicating the received power of the signal(for example, the Discovery response) transmitted from the neighboringterminal to the radio base station 310.

In step 140, the radio base station 310 assigns the D2D radio resourceto be assigned to the UE 10 or the neighboring terminal radio resourceto be assigned to the neighboring terminal (scheduling). Specifically,the radio base station 310 performs radio resource assignmentillustrated in FIGS. 10 and 11.

Furthermore, the radio base station 310 may notify the UE 10 of the factthat the notification of the received power is required (notification ONrequest) after the radio base station 310 notifies the UE 10 of thenotification OFF request, for example, when an usage rate of a radioresource is below a threshold value.

[Second Modification]

A description will be given below of a second modification of the firstembodiment. In the second modification, the UE 10 notifies the radiobase station 310 of the received power in response to the result ofcomparison between the received power of the signal (for example, theDiscovery response) transmitted from the neighboring terminal and athreshold value. For example, when the received power is larger than thethreshold value, the UE 10 notifies the radio base station 310 of thereceived power. On the other hand, when the received power is smallerthan the threshold value, the UE 10 does not notify the radio basestation 310 of the received power.

Furthermore, the UE 10 may notify the radio base station 310 of theidentifier of the neighboring terminal when the received power is largerthan the threshold value. On the other hand, the UE 10 may not notifythe radio base station 310 of the identifier of the neighboring terminalwhen the received power is smaller than the threshold value.

In addition, the threshold value compared with the received power isnotified from the radio base station 310 to the UE 10. The thresholdvalue may be broadcast using a broadcast channel such as SIB and may betransmitted to the UE 10 by using an individual control channel such asPDCCH.

Specifically, as illustrated in FIG. 14, in step 210, the radio basestation 310 notifies the UE 10 of the threshold value to be comparedwith the received power of the signal (for example, the Discoveryresponse) transmitted from the neighboring terminal.

Furthermore, the radio base station 310 notifies the UE 10 of an offsetvalue of the threshold value and/or a determination time range(TimeToTrigger) indicating a time range in which the UE 10 determineswhether the received power is larger than the threshold value, togetherwith the threshold value.

In step 220, the UE 10 detects a neighboring terminal located in theneighborhood of the UE 10. Specifically, the UE 10 broadcasts a signal(the Discovery signal) with predetermined power for detecting aneighboring terminal capable of performing D2D communication. When theneighboring terminal is present, the UE 10 receives a Discovery responsefrom the neighboring terminal as a response to the Discovery signal.

In step 230, the UE 10 determines whether the received power of thesignal (for example, the Discovery response) transmitted from theneighboring terminal is larger than a threshold value. If thedetermination result is “YES”, the UE 10 performs the process in step240. On the other hand, when the determination result is “NO”, the UE 10ends the series of processes.

Furthermore, the UE 10 determines whether the received power of thesignal from the neighboring terminal is larger than the threshold valueon the basis of the offset value and/or the determination time rangewhen the UE 10 is notified of the off set value and/or the determinationtime range.

In step 240, the UE 10 transmits information indicating the receivedpower of the signal (for example, the Discovery response) transmittedfrom the neighboring terminal to the radio base station 310, togetherwith the identifier of the neighboring terminal.

In step 250, the radio base station 310 assigns the D2D radio resourceto be assigned to the UE 10 or the neighboring terminal radio resourceto be assigned to the neighboring terminal (scheduling). Specifically,the radio base station 310 performs radio resource assignmentillustrated in FIGS. 10 and 11.

[Third Modification]

A description will be given below of a third modification of the firstembodiment. In the third modification, the UE 10 notifies the radio basestation 310 of an identifier of a neighboring terminal at apredetermined cycle. The predetermined cycle in which the identifier ofthe neighboring terminal is notified of is notified from the radio basestation 310. The predetermined cycle may be broadcast using a broadcastchannel such as SIB and may be transmitted to the UE 10 by using anindividual control channel such as PDCCH.

Specifically, as illustrated in FIG. 15, in step 310, the radio basestation 310 notifies the UE 10 of the predetermined cycle in which theidentifier of the neighboring terminal is notified of.

In step 320, the UE 10 detects a neighboring terminal located in theneighborhood of the UE 10. Specifically, the UE 10 broadcasts a signal(the Discovery signal) with predetermined power for detecting aneighboring terminal capable of performing D2D communication. When theneighboring terminal is present, the UE 10 receives a Discovery responsefrom the neighboring terminal as a response to the Discovery signal.

In step 330, the UE 10 transmits the identifier of the neighboringterminal to the radio base station 310 according to the predeterminedcycle notified from the radio base station 310. Here, the UE 10 maytransmit information indicating the received power of the signal (forexample, the Discovery response) transmitted from the neighboringterminal to the radio base station 310, together with the identifier ofthe neighboring terminal.

In step 340, the radio base station 310 assigns a D2D radio resource tobe assigned to the UE 10 or a neighboring terminal radio resource to beassigned to the neighboring terminal (scheduling). Specifically, theradio base station 310 performs radio resource assignment illustrated inFIGS. 10 and 11.

[Fourth Modification]

A description will be given below of a fourth modification of the firstembodiment. In the fourth modification, the UE 10 notifies the radiobase station 310 of the identifier of the neighboring terminal inresponse to an instruction (information notification request) from theradio base station 310.

Specifically, as illustrated in FIG. 16, in step 410, the radio basestation 310 notifies the UE 10 of an instruction (informationnotification request) that requests a notification of the identifier ofthe neighboring terminal.

In step 420, the UE 10 detects a neighboring terminal located in theneighborhood of the UE 10. Specifically, the UE 10 broadcasts a signal(the Discovery signal) with predetermined power for detecting aneighboring terminal capable of performing D2D communication. When theneighboring terminal is present, the UE 10 receives a Discovery responsefrom the neighboring terminal as a response to the Discovery signal.

In step 430, the UE 10 transmits the identifier of the neighboringterminal to the radio base station 310 in response to the instruction(information notification request) from the radio base station 310.Here, the UE 10 may transmit information indicating the received powerof the signal (for example, the Discovery response) transmitted from theneighboring terminal to the radio base station 310, together with theidentifier of the neighboring terminal.

In step 440, the radio base station 310 assigns the D2D radio resourceto be assigned to the UE 10 or the neighboring terminal radio resourceto be assigned to the neighboring terminal (scheduling). Specifically,the radio base station 310 performs radio resource assignmentillustrated in FIGS. 10 and 11.

[Fifth Modification]

Hereinafter, a fifth modification of the first embodiment is explained.In the fifth embodiment, the radio base station 310 (hereinafter,referred to as radio base station 310A) notifies another radio basestation 310 (hereinafter, referred to as radio base station 310B) of theidentifier of the neighboring terminal. Here, the radio base station310A may notify the radio base station 310B of information indicatingthe received power of the signal (for example, the Discovery response)transmitted from the neighboring terminal, together with the identifierof the neighboring terminal. Alternatively, the radio base station 310Amay notify the radio base station 310B of the identifier of the UE 10,together with the identifier of the neighboring terminal.

Specifically, as illustrated in FIG. 17, in step 510, the UE 10 detectsa neighboring terminal located in the neighborhood of the UE 10.Specifically, the UE 10 broadcasts a signal (the Discovery signal) withpredetermined power for detecting a neighboring terminal capable ofperforming D2D communication. When the neighboring terminal is present,the UE 10 receives a Discovery response from the neighboring terminal asa response to the Discovery signal.

In step 520, the UE 10 transmits the identifier of the neighboringterminal to the radio base station 310A. Here, the UE 10 may transmitinformation indicating the received power of the signal (for example,the Discovery response) transmitted from the neighboring terminal to theradio base station 310A, together with the identifier of the neighboringterminal.

In step 530, the radio base station 310A notifies the radio base station310B of the identifier of the neighboring terminal. Here, the radio basestation 310A may notify the radio base station 310B of information (thatis, the identifier of the UE 10) indicating the received power of thesignal (for example, the Discovery response) transmitted from theneighboring terminal, together with the identifier of the neighboringterminal.

The radio base station 310A may notify the radio base station 310B ofthe identifier of the neighboring terminal on the basis of notificationfrom the UE 10. The radio base station 310A may notify (respond to) theradio base station 310B of the identifier of the neighboring terminal onthe basis of a request from the radio base station 310B.

In step 540, the radio base station 310A assigns a D2D radio resource tobe assigned to the UE 10 or a neighboring terminal radio resource to beassigned to the neighboring terminal (scheduling). Specifically, theradio base station 310 performs radio resource assignment illustrated inFIGS. 10 and 11.

In step 550, the radio base station 310B assigns a D2D radio resource tobe assigned to the UE 10 or a neighboring terminal radio resource to beassigned to the neighboring terminal (scheduling). Specifically, theradio base station 310 performs radio resource assignment illustrated inFIGS. 10 and 11.

[Sixth Modification]

A description will be given below of a sixth modification of the firstembodiment. In the sixth modification, the radio base station 310specifies a distribution of the UEs 10 present in a cell managed by theradio base station 310 on the basis of the identifier of the neighboringterminal received from the UE 10 and TA (Timing Advance) of the UE 10.

Specifically, first, the radio base station 310 specifies the positionof the UE 10 on a concentric circle around the radio base station 310 onthe basis of the TA of the UE 10. Here, if the TA values of the UEs 10are the same, it is possible to plot the respective UEs 10 on theconcentric circle around the radio base station 310.

Second, the radio base station 310 specifies the distance between the UE10 and the neighboring terminal on the basis of the identifier of aneighboring terminal having the same TA as that of the UE 10 and plotsthe neighboring terminal on the concentric circle. In this stage, theradio base station 310 cannot specify whether the neighboring terminalis present in the clockwise direction or in the counterclockwisedirection on the concentric circle.

Third, the radio base station 310 sequentially specifies the positionsof neighboring terminals of the plurality of UEs 10 present in the cellpresent within the radio base station 310. In this way, the radio basestation 310 can specify the positional relationship of the plurality ofUEs 10 present in the cell present within the radio base station 310with a certain degree of accuracy.

For example, as illustrated in FIG. 18, the plurality of UEs 10 presentin the cell present within the radio base station 310 is plotted ondifferent concentric circles for the respective TA values. In addition,the positions of the neighboring terminals of the plurality of UEs 10present in the cell present within the radio base station 310 aresequentially specified on the basis of the identifier of the neighboringterminal, by transmitting identifier(s) of the neighboring terminal fromeach of the plurality of UEs 10 (for example, transmitting theidentifier of the UE 10-2 being the neighboring terminal from the UE10-1 and transmitting the identifier of the UE 10-3 being theneighboring terminal from the UE 10-2). In this way, the positionalrelationship (in particular, the relative positional relationship) ofthe plurality of UEs 10 present in the cell present within the radiobase station 310 is specified with a certain degree of accuracy.

[Seventh Modification]

A description will be given below of a seventh modification of the firstembodiment. In the seventh modification, the radio base station 310specifies a distribution of UEs 10 present in the cell managed by theradio base station 310 on the basis of the identifier of the neighboringterminal received from the UE 10 and the received power of the signal(for example, the Discovery response) transmitted from the neighboringterminal.

Specifically, as illustrated in FIG. 19, a case where UE 10 _(A), UE 10_(B), and UE 10 _(C) are present in the cell managed by the radio basestation 310 is illustrated. Here, the radio base station 310 specifiesthe UE 10 _(B) and UE 10 _(C) as the neighboring terminals of the UE 10_(A), specifies a distance 2 between the UE 10 _(A) and UE 10 _(B) onthe basis of the received power of a signal that the UE 10 _(A) receivesfrom the UE 10 _(B), and specifies a distance 1 between the UE 10 _(A)and UE 10 _(C) on the basis of the received power of a signal that theUE 10 _(A) receives from the UE 10 _(C). Similarly, the radio basestation 310 specifies the distance 2 between the UE 10 _(A) and UE 10_(B) and the distance 3 between the UE 10 _(B) and UE 10 _(C) andspecifies the distance 1 between the UE 10 _(A) and UE 10 _(C) and thedistance 3 between the UE 10 _(B) and UE 10 _(C).

In this way, it is possible to specify the positional relationshipbetween the UE 10 _(A), UE 10 _(B), and UE 10 _(C), and the distributionof the UEs 10 present in the cell managed by the radio base station 310is specified.

[Eighth Modification]

A description will be given below of an eighth modification of the firstembodiment. In the eighth modification, as illustrated in FIG. 20, theradio base station 310 specifies the position of the UE 10 on the basisof TA and the arrival angle of TA. Specifically, the radio base station310 specifies the distance between the radio base station 310 and the UE10 on the basis of TA and specifies the direction in which the UE 10 ispresent on the basis of the arrival angle of TA. In this way, theaccuracy of specifying the position of the UE 10 present in the cellmanaged by the radio base station 310 is improved.

[Other Embodiments]

The present invention is explained through the above embodiment, but itmust not be understood that this invention is limited by the statementsand the drawings constituting a part of this disclosure. From thisdisclosure, various alternative embodiments, examples, and operationaltechnologies will become apparent to those skilled in the art.

The embodiment has described the case in which the two UEs 10communicate with each other in the D2D communication. However, theembodiment is not limited thereto. Specifically, in the D2Dcommunication, three or more UEs 10 may communicate with one another.

Although it is not particularly mentioned in the embodiment, bycombining at least two of the sixth to eighth modifications, theaccuracy of specifying the distribution of the UEs 10 present in thecell managed by the radio base station 310 may be improved.

In the embodiment, the radio base station 310 specifies the distributionof the UEs 10 present in the cell managed by the radio base station 310.However, the embodiment is not limited thereto. The distribution of theUEs 10 present in the cell managed by the radio base station 310 may bespecified by a node (for example, the network device 330) higher thanthe radio base station 310. In such a case, the information such as theidentifier of the UE 10, the TA of the UE 10, the identifier of theneighboring terminal, and the received power of the signal (for example,the Discovery response) transmitted from the neighboring terminal isnotified from the radio base station 310 to the upper node.

In the embodiment and the first modification to the fifth modification,the UE 10 notifies the radio base station 310 of the identifier of theneighboring terminal and the information indicating the received powerof the signal (for example, the Discovery response) transmitted from theneighboring terminal in response to the Discovery response received fromthe neighboring terminal. However, the embodiment is not limitedthereto. Specifically, the neighboring terminal may notify the radiobase station 310 of the identifier of the UE 10 included in theDiscovery signal and the information indicating the received power ofthe signal (for example, the Discovery signal) transmitted from the UE10 in response to the Discovery signal received from the UE 10 (the D2Dterminal).

In the embodiment, the threshold value compared with the received powerof the signal (for example, the Discovery response) transmitted from theneighboring terminal is notified from the radio base station 310.However, the embodiment is not limited thereto. The threshold valuecompared with the received power of the signal (for example, theDiscovery response) transmitted from the neighboring terminal may bedetermined in advance.

Although it is not particularly mentioned in the embodiment, thedistribution of the UEs 10 present in the cell managed by the radio basestation 310 is useful when used in SON (Self Organizing Network) or MDT(Minimization of Drive Tests). Moreover, the distribution of the UEs 10present in the cell managed by the radio base station 310 may be used inradio resource scheduling.

In the embodiment, the UE 10 (the UE 10A or UE 10B) determines a changein the communication state of the user data communicated directlybetween the UE 10A and UE 10B. However, the embodiment is not limitedthereto. Specifically, the radio base station 310 may determine thechange in the communication state of the user data communicated directlybetween the UE 10A and UE 10B.

In such a case, on the basis of the D2D control signal received from theUE 10 (the UE 10A or UE 10B), the radio base station 310 determines thechange in the communication state of the user data communicated directlybetween the UE 10A and UE 10B. The D2D control signal is a signalindicating the transmission power used for the communication of the userdata or a signal indicating the modulation and coding scheme used forthe communication of the user data.

In the embodiment, the change in the communication state of the userdata communicated directly between the UE 10A and UE 10B is mainlydetermined on the basis of the transmission power used for thecommunication of the user data or a modulation and coding scheme usedfor the communication of the user data. However, the embodiment is notlimited thereto. The change in the communication state of the user datacommunicated directly between the UE 10A and UE 10B may be determined onthe basis of whether a block error rate, a packet error rate, and apredetermined QoS are satisfied, and any one of the CQI and theprocessing load of the UE 10.

For example, in the case in which the UE 10 (the UE 10A or UE 10B)determines the change in the communication state of the user data, theD2D control signal indicates at least one of: a signal indicating that ablock error rate related to the D2D communication exceeds the thresholdvalue; a signal indicating that the block error rate related to the D2Dcommunication becomes less than the threshold value; a signal indicatingthat a packet error rate related to the D2D communication exceeds thethreshold value, a signal indicating that the packet error rate relatedto the D2D communication becomes less than the threshold value; a signalindicating whether predetermined QoS related to the D2D communication issatisfied, a signal indicating that CQI related to the D2D communicationbecomes less than the threshold value; a signal indicating that the CQIrelated to the D2D communication exceeds the threshold value; a signalindicating that a processing load of the UE 10 exceeds the thresholdvalue, and a signal indicating that the processing load of the UE 10becomes less than the threshold value.

Alternatively, in the case in which the radio base station 310determines the change in the communication state of the user data, theD2D control signal indicates at least one of: a signal indicating theblock error rate related to the D2D communication; a signal indicatingthe packet error rate related to the D2D communication; a signalindicating the CQI related to the D2D communication; and a signalindicating the processing load of the UE 10.

In the embodiment, an entity that switches the first assignment mode andthe second assignment mode is mainly the UE 10 (the UE 10A or UE 10B).However, the embodiment is not limited thereto. For example, the entitythat switches the first assignment mode and the second assignment modemay be the radio base station 310. In such a case, the radio basestation 310 transmits a signal for requesting the switching of the firstassignment mode and the second assignment mode to the UE 10 (the UE 10Aor UE 10B).

Particularly not mentioned in the embodiment, it is possible to providea program for causing a computer to execute each process performed bythe UE 10 (the UE 10A or UE 10B). Furthermore, the program may berecorded on a computer-readable medium. By using the computer-readablemedium, it is possible to install the program in a computer.Furthermore, the computer-readable medium recording the program thereonmay include a non-transitory recording medium. The non-transitoryrecording medium is not particularly limited. For example, thenon-transitory recording medium may include a recording medium such asCD-ROM or DVD-ROM.

Alternatively, it is also possible to provide a chip configured by amemory for storing a program for performing each process performed bythe UE 10 (the UE 10A or UE 10B), and a processor for executing theprogram stored in the memory.

In addition, the entire content of U.S. Provisional Application No.61/676,793 (filed on Jul. 27, 2012) and U.S. Provisional Application No.61/676,802 (filed on Jul. 27, 2012) are incorporated in the presentspecification by reference.

INDUSTRIAL APPLICABILITY

As described above, in the mobile communication system and the mobilecommunication method according to the present invention, the process forthe radio terminal to discover another terminal could also be appliednot only to the D2D communication but also to another process, and thusis available for a radio communication field.

The invention claimed is:
 1. A radio terminal, comprising: a receiver; acontroller; and a transmitter; wherein the receiver is configured toreceive information on radio resources for direct communication from abase station by broadcast, the controller is configured to perform thedirect communication by use of a radio resource autonomously selectedamong the radio resources; the receiver is configured to directlyreceive a radio signal from another radio terminal during performing thedirect communication by use of the radio resources; the controller isconfigured to compare received power of the radio signal with athreshold value, wherein the threshold value is not a received powervalue from another radio terminal; and the transmitter is configured to:notify the base station of information on the received power in responseto the received power being larger than the threshold value; and notnotify the base station of the information on the received power inresponse to the received power being smaller than the threshold value.2. The radio terminal according to claim 1, wherein the transmitter isconfigured to notify the base station of the received power, togetherwith an identifier of the another radio terminal.
 3. The radio terminalaccording to claim 1, wherein the receiver is configured to receive aninstruction from the base station, and the transmitter is configured tonotify the base station of an identifier of the another radio terminalin response to the instruction.
 4. A mobile communication method,comprising: transmitting, by a base station, information on radioresources for direct communication by broadcast; receiving, by a radioterminal, the information on radio resources for direct communication,from the base station; performing, by the radio terminal, the directcommunication by use of a radio resource autonomously selected among theradio resources; directly receiving, by the radio terminal, a radiosignal from another radio terminal during performing the directcommunication by use of the radio resources; comparing, by the radioterminal received power of the radio signal with a threshold value,wherein the threshold value is not a received power value from anotherradio terminal; notifying, by the radio terminal, the base station ofinformation on the received power in response to the received powerbeing larger than the threshold value; and not notifying, by the radioterminal, the base station of the information on the received power inresponse to the received power being smaller than the threshold value.5. The mobile communication method according to claim 4, furthercomprising: notifying the base station of an identifier of the anotherradio terminal; and controlling, by the base station, a radio resourceto be assigned to the radio terminal on the basis of the identifier ofthe another radio terminal.
 6. A processor of a radio terminal, theprocessor communicatively coupled to a memory and configured to: receiveinformation on radio resources for direct communication from a basestation by broadcast; perform the direct communication by use of a radioresource autonomously selected among the radio resources; directlyreceive a radio signal from another radio terminal during performing thedirect communication by use of the radio resources; compare receivedpower of the radio signal with a threshold value, wherein the thresholdvalue is not a received power value from another radio terminal; andnotify the base station of information on the received power in responseto the received power larger than the threshold value; and not notifythe network of the received power in response to the received powerbeing smaller than the threshold value.
 7. The radio terminal accordingto claim 1, wherein the receiver is configured to receive information ona determination time range indicating a time range in which the radioterminal determines whether the received power is larger than thethreshold value.